The present invention relates generally to cellular and wireless communication and more particularly to a system and method for communicating control and user information in a cellular packet data communication system
Recently, there has been a trend in the telecommunication community to focus more and more on wireless packet data communication rather than circuit switched voice communication. With the tremendous increase of Internet users, it is believed that the packet switched communication will soon increase more and become, larger than the circuit switched voice communication that today dominates the e.g., the cellular communication. Cellular communication system manufacturers and operators are therefore looking for solutions to integrate their circuit switched services with wireless packet switched services that can provide reliable and more spectrum efficient connections for packet switched users, e.g., Internet users. This trend has made different types of packet switched communication system evolutions flourish. One of the more well known packet switched cellular systems in the telecommunications community, even though it is not yet launched, is the extension of the present GSM cellular communication system, called GPRS (General Packet Radio Services).
GPRS is a packet switched system that uses the same physical carrier as the present GSM cellular communication system and is designed to coexist and provide the same coverage as GSM. GPRS radio interface is thus based on a TDMA structured system with 200 kHz carriers divided into eight timeslots with GMSK (Gaussian Minimum Shift Keying) modulation. The multiplexing is such that each timeslot can typically serve a number of users and one user can also be allocated more than one timeslot to increase its user throughput of data over the air. The GPRS specification includes a number of different coding schemes to be used dependent on the quality of the radio carrier. with GPRS, data rates well over 100 kbps will be possible.
Recently there has also been a development and standardization effort to further increase the data throughput to GPRS, by introducing a higher level modulation than what is used for GSM and GPRS, i.e., GMSK. The enhancement of GPRS is called EGPRS (Enhanced GPRS) and employs an 8PSK (8 Phase Shift Keying) modulation format which will increase the data rates to over 384 kbps. Additional coding schemes are developed for this 8PSK that will make an efficient link adaptation possible. Link adaptation is a functionality that allows adaptation in terms of coding and modulation with respect to present signal quality. In poor radio conditions, a robust coding and GMSK modulation is selected whereas in good radio Conditions, a less robust coding and 8PSK modulation is used. GPRS (and the extensions thereof, also has a backward error correction functionality in that it can request retransmissions of erroneously received blocks. This mechanism is called ARQ (Automatic Repeat reQuest) and is a well known mechanism in the art.
The GPRS is thus developed with the aim to introduce packet switched access in GSM system environments. Even though some features are duplicated for GPRS, e.g., specific packet control channels are defined, it is still dependent upon the existence of GSM circuit switched control channels.
For example GPRS may have its own Packet Broadcast Channels (PBCCH) and Packet Paging Channels (PPCH) but for cell reselection purposes, it is still dependent on listening to a GSM broadcast carrier. The reason for this is that a GPRS system does not provide any synchronization burst or frequency correction burst on the physical channels that carry the packet control channels, as presently standardized. This means that a GPRS system cannot exist by itself, it is dependent upon a GSM circuit switched control channel.
There is thus a problem today, to be able to run a stand-alone GPRS system without having to also implement a circuit switched mode,
Recent development for another TDMA based cellular system, the cellular communication system compliant to the IS-136 standard, below referred to as TDMA/136 has indicated a need for a high throughput packet data system to be integrated with the TDMA/136 circuit switched mode. Several companies in the telecommunication industry have come to an agreement that a high-speed packet switched mode in a TDMA/136 system could be based on GPRS and the extensions thereof. This will allow for possibilities of providing bit rates up to 384  less than bps on 200 kHz carriers with GMSK and 8PSK modulation, like that of GSM/GPRS.
This integration does not, however, come without a cost. First and foremost, the TDMA/136 carriers have a bandwidth of only 30 kHz, to be compared with GSM/GPRS carriers of 200 kHz. This means that operators that want to introduce GPRS have to allocate 200 kHz for each GPRS carrier. Adding that some frequency reuse is needed for providing coverage and for limiting co-channel interference, this means that a TDMA/136 operator have to allocate a substantial amount of spectrum and thereby allocate bandwidth to GPRS that could otherwise be used for 30 kHz circuit switched channels. This has put requirements on the implementation of GPRS on 200 kHz carriers in TDMA/136 system. It should be possible to introduce GPRS within a bandwidth requirement below 1 MHz. Taking possible frequency reuse patterns and guard bands between 30 kHz carriers and 200 kHz carriers into consideration, this means that a 1/3 frequency reuse pattern must be deployed. A 1/3 frequency reuse pattern means, e.g., that all base station sites transmit in three sectors and with a different set of physical channels in each sector.
A physical channel can be a frequency or a timeslot or a combination thereof. Of course, if other systems than TDMA systems are considered, e.g., CDMA systems, a physical channel can also include combinations of a certain code pattern. Logical channels can be transmitted on one or more physical channels. A logical channel is defined by its content, e.g., control channels, traffic channels, broadcast channels etc.
Reuse patterns are introduced in cellular systems such that one can reuse the same frequencies in different cells. This should typically be done in a way such that the co-channel interference caused by base stations transmitting on the same frequency in closely located cells is limited. The higher the reuse, the better the carrier to interference ratio for an exemplary condition. A 1/3 reuse is illustrated in FIG. 1.
GPRS channels typically have different levels of robustness depending on the type of logical channel being transmitted. In a packet data system, reliance on retransmission possibilities can allow a quite high error rate and at least for this reason, the reuse for user data traffic channels can be kept quite low. A data traffic channel can be deployed in a 1/3 reuse whereas packet common control channels are not robust enough to be allocated in a 1/3 reuse, since the same retransmission possibilities are not used for these channels. A 3/9 reuse is recommended for packet data control channels. A 3/9 reuse means that there are 9 different sets of physical channels divided over three base station sites, where each site transmit in three sectors.
Considering the spectrum requirement of 1 MHz and a possible reuse requirement of 3/9 for packet common control channels, it is not possible, by just dividing the physical channels into different frequencies, to achieve such a reuse pattern. Only three-four 200 kHz frequencies can be allocated on 1 MHz, since some 2xc3x97100 kHz are needed as guard band between different types of carriers, e.g., 30 and 200 kHz carriers. Therefore, some other strategy is needed.
In patent application document WO 97/12489 with Pacific Communication Sciences Inc. as assignee is described a time sharing method and apparatus for frequency reuse in cellular communication systems. Frequency reuse is enhanced by synchronizing cell transmit/receive base stations in a cellular system to a common time base, an then sharing the available frequencies via allocated time sits.
In patent application document WO 98/31110 with Nokia Telecommunications as assignee, a method for improving system capacity is described. Nearby base stations using the same carrier do not send a broadcast time slot substantially simultaneously.
There are ways to increase a reuse pattern without having to allocate a higher frequency reuse. In a synchronized system, an overlaid timeslot reuse is also possible. The WO 98/31110 application is about broadcast messages in a circuit switched communication system.
It is found that the techniques described in these patent applications can be substantially improved to ensure control signalling message arrival in a receiving radio station and that additional actions can be taken in a synchronized system to ensure reliable reception of control signalling messages.
There are thus possibilities, in a further improved solution, to receive control signalling in a system that employs a lower frequency reuse than what is required by the control channels to be transmitted therein.
In another aspect it is to be noted that to be able to introduce 200 kHz GPRS carriers with 8PSK and GMSK modulation in a TDMA/136 system, the GPRS functionality that relies on a GSM control channel needs to be considered. It will not necessarily be the case that a GSM control channel is desired in a TDMA/136 system, since this system may not have a GSM circuit switched mode at all. As mentioned above, the channels that are necessary for a GPRS system, but transmitted in a GSM circuit switched mode, are the synchronization channel (SCH) and the Frequency Correction channel (FCCH) in GSM. GPRS systems rely on the capability of listening to this information. Therefore, it would be advantageous with a method for introducing synchronization and frequency correction possibilities in a GPRS system without also having to implement GSM circuit switched mode and the control channels associated therewith.
In one aspect of the present invention, a method and system is described, that can handle the above described deficiencies and provide a sufficient reuse pattern by introducing additional requirements on the control channel allocation. This method is based on a timeslot and frequency reuse in a base station synchronized system, such that a base station site is allocated certain timeslot or timeslots, herefrom called time group, on a frequency for control channel allocation. In another base. station site, using the same frequency, a different time group is allocated, such that the control channels are not allocated at the same time in closely located base station sites. In addition, to be able to handle propagation delays, different time groups do not include timeslots adjacent to a different time group. There is thus a minimum of one timeslot between two time groups to be able to handle propagation delays and avoid overlap of control channel symbols from different time groups.
In another aspect of the present invention, a first base station site that is allocated a certain frequency and time group to transmit control channel messages on the timeslots included in the time group. A second base station site is allocated the same frequency and another time group and could in theory transmit non-control channel messages on the timeslots coinciding with the first base station site""s control channel allocated time group. In this aspect of the present invention, no transmission occurs in the timeslots coinciding with the first base stations time group for control channels. This improvement will ensure a more reliable reception of control channel signalling in all base station sites
In yet another aspect of the present invention, a synchronization channel is introduced in a set of packet common control channels, such that, in that respect, a system compliant to the GPRS specification will not be dependent upon the implementation of a GSM circuit switched control channel. To be able to distinguish this new synchronization channel from the synchronization channel used in circuit switched GSM, a new coding is suggested. This new coding will allow a receiving unit to distinguish between different types of synchronization channels by reading a specific bit pattern value that are not present in the existing synchronization channel in circuit switched GSM.